Propagation of waves excited by localized episodic heating in the tropical troposphere and their effect on the middle atmosphere are investigated numerically with a global primitive-equation model in which a realistic radiation scheme for the middle atmosphere is incorporated. Equinoctial initial states with two opposite phases of the quasi-biennial oscillation (QBO) are used for comparison of the propagation and the effect of the waves. Time evolutions of the responses of the equinoctial initial states are not much different from the linear responses of a resting atmosphere obtained by Horinouchi and Yoden (1996). If the duration of the heating is small (less than about a day), Eliassen-Palm (EP) flux in the middle atmosphere is mainly due to gravity waves including Kelvin waves, while if the duration is large (more than about a day), it is mainly due to Kelvin waves, Rossby waves, and Rossby-gravity waves. In the westerly-shear phase of the QBO, westerly acceleration comparable to or a little smaller than that required by the QBO is obtained for wide range of heating parameters, while in the easterly-shear phase, realistic easterly acceleration cannot be obtained if the heating events have large time and horizontal scales (more than about a day and a few thousand kilometers). Gravity waves propagating into the low- and mid-latitude mesosphere are affected by the QBO. The difference in the divergence of the EP flux due to gravity waves may explain the observed quasi-biennial variations in the low- and mid-latitude mesosphere. Excitation of the global normal mode 5-day wave is sensitive to the QBO phase. Since such a QBO modulation has not been observed, this result implies that the source of the 5-day wave in the real atmosphere is not or not only in the tropics, but a considerable portion of the wave is excited outside the tropics.
Decadal and interannual variations in the global atmosphere and ocean are investigated using the global sea surface temperature (SST), sea-level pressure, surface winds, precipitation and Northern Hemisphere 500hPa geopotential height data for 30 to 50 years. The most dominant interannual variation in the global atmosphere and ocean is associated with El Niño and Southern Oscillation (ENSO) phenomena. Two dominant decadal modes are obtained. The first mode mainly dominates over the entire Pacific Ocean and changed abruptly in the late 1970s. The spatial pattern of the tropical SST after 1978 contains large positive anomalies over most of the tropical ocean, particularly over the eastern Pacific. Negative SST anomalies are found in the mid-latitudes over the North and the South Pacific. The sea-level pressure in the tropics shows negative anomalies in the eastern Pacific, but positive anomalies in the western Pacific and Indian Oceans. There exist westerly anomalies in the equatorial western-central Pacific, indicating the weakening of the tropical Walker circulation. The surface precipitation increased in the tropical centraleastern Pacific where large increases of SST exist. Composite and singular value decomposition (SVD) analyses demonstrate that the Pacific/North American (PNA) pattern was strengthened and associated with this first decadal mode. It is suggested that the enhanced tropical convection due to the increase of SST over the tropical central-eastern Pacific may strengthen the PNA pattern. The second decadal mode has dominant SST anomalies mainly in the Northern Hemisphere midlatitudes. The Northern Hemisphere 500hPa height field associated with this mode shows a dominance of a north-south seesaw between middle latitudes and higher latitudes. A north-south dipole structure in the western Atlantic is especially noticeable. Large positive anomalies extending from East Asia to the North Pacific are also significant. The second mode has a time scale of about 15 years and changed its sign from negative to positive around 1987. Linear trends for SST, sea-level pressure, surface winds and 500hPa height are also investigated. Significant positive trends of SST are found over the most tropical regions and negative SST trends exist in the North Pacific. There exist large trends of sea-level pressure and surface winds over the North Atlantic. Easterlies are strengthened over most of the tropics. The PNA pattern in the 500hPa height field has tended to intensify during the last 50 years.
CO2-induced climate change related to the effect of cumulus convection on the vertical profile of water vapor was studied using a vertical one-dimensional radiative convective model with the Arakawa-Schubert cumulus parameterization that accounts for the drying effect of cumulus convection. Annual mean tropical solar forcing is used because cumulus convection is most active in the tropics. Since moisture and heat transport by large-scale motion (LS forcing) are important to moisture and heat balance in the tropics, two types of doubling CO2 experiments were conducted with and without considering LS forcing. The increase in specific humidity is small above the middle troposphere without LS forcing and the surface temperature increase is much smaller than that obtained in a three-dimensional CO2 experiment. When LS forcing is specified, however, water vapor increases significantly throughout the troposphere and the surface temperature increase is enhanced to a similar degree as in a three-dimensional experiment. LS forcing of heat enhances cumulus convection under control CO2 concentration, while LS forcing of moisture contributes to enhance changes due to CO2 doubling. Thus, the magnitude of cumulus convection response to change in radiative forcing is very sensitive to LS forcing. This means that it is important to incorporate the effects of moisture and heat transport by large-scale motion when investigating cumulus convection response to climate changes.
The Rossby wave propagation in various basic flows is studied, based on the numerical time-integration of a barotropic model on the sphere. First, the propagation in idealized basic flows is examined. When the zonally uniform basic flow has a strong jet, the jet stream acts like a waveguide for the Rossby wave. When the basic flow has a component of zonal wavenumber 1 (or 2), the Rossby wave emanating from the entrance region of the jet rapidly propagates eastward to the jet exit region. Then, the wave becomes stagnant and its amplitude is increased in the exit region. When the wave component superposed in the basic flow is strong, the basic flow has barotropically unstable modes. Nevertheless, this instability is weak so that the propagation patterns are not influenced by it. The barotropic conversion of kinetic energy from the basic field is evaluated to be important in this wave amplification in the jet exit region. Then, it can be understood that this wave amplification is produced both by the stagnation of wave energy from the energy source and by the subsequent barotropic conversion in the jet exit region. Second, the wave propagation is examined for monthly-mean basic flows derived from wintertime observational data. It is found that the propagation property of the Rossby wave is highly dependent on the basic wind distribution. In one case (December 1986) the Rossby wave, which has propagated eastward through the Asian jet, is blocked and becomes stagnant near the jet exit region. In contrast, in the other case (January 1984) near the jet exit the propagation path splits into eastward and equatorward paths.
Difficulties in using conventional Fourier spectral analysis to explore the temporal variation of CO2 short-term fluctuations have indicated the need for some new statistical techniques. It is suggested here that the Multiresolution Fourier Transform (MFT) can be used for interpreting such fluctuations. The technique is adapted from wavelet theory in signal processing. To demonstrate the usefulness of this technique, some examples of analytic fluctuation are first examined. The examples suggest that this technique might be useful for analysis of fluctuations that exhibit locality in both frequency and time. MFT is then applied to study the CO2 fluctuations measured at Alert from 1988 to 1995. The results show that the CO2 concentration over Alert has two dominant regimes of quasi-periodic short-term fluctuation: 1)20-50 day intraseasonal fluctuation and 2) 6-14 day synoptic fluctuation. Both the amplitude and frequency of these fluctuations are strongly modulated by the seasonal cycle and the largest amplitude appears during winter time. In addition, these short-term fluctuations have significant interannual variability, especially for the synoptic fluctuation.
An ensemble climate simulation experiment has been conducted using the Japan Meteorological Agency (JMA) global model to study the SST-forced atmospheric variability and potential predictability of seasonal mean fields. The ensemble consists of three model integrations each for the same 34-year period. The three integrations use the same observed SST for the period 1955-1988 but start from different atmospheric initial states. The variance ratios of the SST-forced variability to the total variability of the seasonal mean fields are computed. The ratios are considered to represent “potential predictability” (possible maximum predictability when SST is perfectly predicted). The variance ratios of pressure fields are generally high (50-90%) in the tropics but low (less than 30%) in the extratropics, suggesting that the potential predictability of the seasonal mean fields is high in the tropics but low in the extratropics. The variance ratios of precipitation take a wide range of values within the tropics from 74% for N. E. Brazil rainfall to 31% for Indian summer monsoon rainfall, indicating large regional differences in the potential predictability of seasonal mean rain-fall within the tropics. The variance ratio of globally-averaged global mean land surface air temperature is high (66%) but the ratio is low (less than 30%) for the seasonal mean local surface air temperature over most land area. This suggests that the potential predictability of local land surface air temperature is low.
Yamase Air-Flow (YAF) is an outflow of the summertime maritime polar airmass over the North Pacific toward the San-Riku district of Japan (SRJ) and forms along the southern or southeastern edge of an anticyclone developed over the Okhotsk Sea (Okhotsk Sea high). In most summers, the Okhotsk Sea high appears intermittently, but the high and associated YAF persisted for an abnormally long period between mid July and mid August in 1993. Due to the continuous YAF, an abnormal low temperature accompanied with synoptic-scale (several or ten days) variation was observed in the SRJ during this period. We studied mechanisms of the AirMass Transformation (AMT) of the YAF in 1993 over the western North Pacific for both relatively warm and cool spells of the variation. The temperature of the YAF at the coast of the SRJ varied over a narrow range around the offshore Sea Surface Temperature (SST). The temperature sometimes exceeded the SST plus 3°C only for a short period in the warm spells but never fell below the SST minus 3°C in the cool ones. Namely, the offshore SST controlled the lower limit of the YAF's temperature. The temperature variation was related to trajectories of air-parcels in the YAF over the western North Pacific. The YAF was directed southwestward toward the SRJ from Kuril Islands during the cool spells, while directed southwestward over the North Pacific and turned northwestward off the SRJ during the warm spells. Since SST gradient is large off the SRJ, the meridional direction of the YAF largely affected the AMT of the YAF. Trajectories of the YAF were related to the southeastward extension of the Okhotsk Sea high into the North Pacific, which was stronger in the warm spells. During the cool spells, an atmospheric mixed layer accompanied with low-level clouds appeared in the YAF at the coast of the SRJ. The ocean supplied sensible heat of ∼30Wm-2 and latent heat of ∼80Wm-2 to the YAF on its way from the neighborhood of Kuril Islands to the SRJ. This heating compensated strong radiative cooling at the top of the low-level clouds (∼-70Wm-2) and kept the temperature of the YAF higher than the offshore SST minus 3°C. During the warm spells, a stable layer attached to the sea surface accompanied with low-level clouds appeared in the YAF. Off the SRJ, the temperature of the YAF exceeded the offshore SST by several degrees and the YAF was cooled (-10∼-20Wm-2) and lost its moisture (0∼-20Wm-2) at its bottom. The low-level clouds also contributed to the cooling of the YAF through the radiative process (∼-70Wm-2). The cooling of the YAF on its way to the SRJ, however, sometimes may not have been enough to adjust its temperature to the SST when northwestward advection of the YAF off the SRJ was rapid or the radiative cooling was weakened by upper-level clouds. Even during the cool spells, the AMT of the YAF was much weaker than that of the Asian winter monsoon breaking out over the Kuroshio area. In the AMT of the winter monsoon, radiative cooling at the top of the low-level clouds can be neglected in comparison with the dominant heat supply from the ocean. In the AMT of the YAF, on the other hand, the radiative cooling was comparable to the oceanic heating or cooling.
A light balloon-borne aerosol sampler was developed to collect individual stratospheric aerosol particles in order to analyze the composition by electron microscopy. The sampler, which is 917g in weight, consists of a two-stage impactor, a pump and a control unit. The sampler can be easily lifted using a rubber balloon of 3kg in weight and be recovered in rural areas by predicting the accurate falling points. This sampler can fix the position of an electron microscopic grid just below the impactor jet during each collection. During an observation, 16 samples in each impactor stage can be collected. Aerosol samples from the ground to the lower stratosphere were obtained by using this sampler in August 1994 over Tsukuba, Japan. Results on the observation of stratospheric particles showed that sulfuric acid particles were dominant in particles of radius between 0.03 and 1μm. As compared with a single-stage impactor used in our previous observations, the use of the two-stage impactor could reduce the overlap of the satellite droplet rings formed by sulfuric acid particles on the collecting surface which masked smaller particles. The samples collected by this sampler are useful to examine the composition of individual particles in a wide size range by electron microscopy.